Dc microgrid for interconnecting distributed electricity generation, loads, and storage

ABSTRACT

A device includes an energy unit coupled to an energy device and adapted to couple a pair of split DC rails. A controller senses the voltage on the DC rails and regulates its output current response by means of an autonomous current response that creates the aggregate effect of controlling the rail voltage in cooperation with other units coupled to the DC rails. A system includes multiple such devices coupled to split DC rails.

RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 61/490,563 (entitled Universal interconnection bus for power anddata transfer from distributed energy devices, filed May 26, 2011) whichis incorporated herein by reference.

BACKGROUND

As the world shifts to cleaner sources of energy, electricity generationis becoming increasingly distributed, in response to the geographicallydispersed nature of the available clean resources. Also, large-scaleelectric storage capability will be needed, due to the varyingavailability of natural energy sources.

The function of an electrical power grid is to transmit electrical powerfrom its sources to its loads. Its origins date to the 1880s when ThomasEdison established the first direct current (DC) distribution grid,which was soon replaced by Tesla's alternating current (AC) grid. ACtransmission won out because of the ease of increasing AC voltage withlow-frequency transformers for long-distance transmission, andsubsequently transforming back to lower voltage at the point of use. Theexisting AC infrastructure (“the grid”) works well for large centralizedpower plants with distributed loads, but is not well-suited to supportdistributed power production or electrical energy storage. Among otherlimitations, the existing AC grid has no built-in provisions forcommunication, for instance to communicate the real-time availability ofenergy relative to demand.

Advances in power electronics are enabling efficient and inexpensive DCpower conversion, while rising materials prices (notably copper) add tothe cost of conventional AC power conversion. This is especially true inthe case of distributed sources such as solar, wind, and fuel cells,since these sources are either fundamentally DC in nature or must beconverted to DC before they can be converted to grid-compatible AC. Mostmeans of electrical storage are also fundamentally DC in nature, as arenearly all modern loads (with the exception of induction motors). Thepresent requirement of converting the inputs/outputs of these devices toAC for interconnection reduces their efficiency and increases theircosts.

SUMMARY

A DC electricity distribution network or microgrid is disclosed,providing a means to interconnect disparate electrical loads, storageand generation. In one embodiment, the network comprises a split-voltageparallel DC bus and a plurality of converter units attached to the bus,where the converter units have a real-time voltage-level signaling meansfor maintaining bus stability, and a powerline-carrier communicationmeans for transmitting and receiving status and performance data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block schematic diagram of a local energy distributionsystem according to an example embodiment.

FIG. 1B is a block schematic diagram of an energy unit according to anexample embodiment.

FIG. 1C is a block schematic diagram of a further energy unit accordingto an example embodiment.

FIG. 2A is a graph illustrating operation of a source unit according toan example embodiment.

FIG. 2B is a graph illustrating operation of a load unit according to anexample embodiment.

FIG. 2C is a graph illustrating operation of an energy storage unitaccording to an example embodiment.

FIG. 3 is a block schematic diagram illustrating the disconnect circuitdetails of a source converter according to an example embodiment.

FIG. 4 is a block schematic diagram of a local energy distributionsystem to an example embodiment.

FIG. 5A is a graph illustrating a voltage controlled power regulationcurve of an energy unit according to an example embodiment.

FIG. 5B is a graph illustrating a voltage controlled power regulationcurve for a power storage unit according to an example embodiment.

FIG. 5C is a graph illustrating a voltage controlled power regulationcurve for an additional load energy unit according to an exampleembodiment.

FIG. 6A is a block schematic diagram illustrating a model of one or moresources connected in parallel with one or more sinks according to anexample embodiment.

FIG. 6B is a block schematic diagram illustrating an alternative modelwhere one of the sinks has been replaced with an equivalent small signalimpedance according to an example embodiment.

FIG. 6C is a block schematic diagram illustrating a simplified sectionused to calculate the system dynamics according to an exampleembodiment.

FIG. 7 shows an example of magnitude and phase keepout areas forcomponent source and sink impedance according to an example embodiment.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanyingdrawings that form a part hereof, and in which is shown by way ofillustration specific embodiments which may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that structural, logical andelectrical changes may be made without departing from the scope of thepresent invention. The following description of example embodiments is,therefore, not to be taken in a limited sense, and the scope of thepresent invention is defined by the appended claims.

The functions or algorithms described herein may be implemented insoftware or a combination of software and human implemented proceduresin one embodiment. The software may consist of computer executableinstructions stored on computer readable media such as memory or othertype of storage devices. Further, such functions correspond to modules,which are software stored on a storage device, hardware, firmware or anycombination thereof. Multiple functions may be performed in one or moremodules as desired, and the embodiments described are merely examples.The software may be executed on a digital signal processor, ASIC,microprocessor, or other type of processor operating on a computersystem, such as a personal computer, server or other computer system.

For purposes of this description an energy unit is any electronic devicethat includes a power converter that connects via suitable contacts to adirect current (DC) source/sink to another electric source or sinkincluding DC/DC, DC/AC, AC/DC voltage converters or current converters.An energy device is an AC or DC source or sink of electric power.Examples include solar panels, wind turbines, batteries, utility grids,lighting and other loads. A bus is a pair of DC conductor rails.

Referring to FIG. 1A, in one embodiment a local energy distributionsystem is provided, comprising positive rail 101 and negative rail 102,which carry DC current at substantially equal and opposite electricpotential of approximately 180-210V relative to ground 103, and servingto connect one or more source energy units 110, 120 to the AC grid 190,through an inverter energy unit 140, and optionally to storage energyunit 130, which connects the battery bank energy device 131 to the DCrails. Additional energy units may be connected in parallel, forinstance to serve DC loads. The substantially equal and oppositeelectric potential enables a simplified inverter design by providing theminimum voltage required to make the positive and negative going outputwave shapes while not being such a high voltage as to require moreexpensive switching components. The voltages on each rail need not beequal and opposite, but may vary by up to ten percent or so in someembodiments. In further embodiments, the potential of each rail orconductor relative to ground potential is in the range of +/−180-210volts.

Referring to FIG. 1B, energy unit 110 accepts electric power from anenergy device, for instance PV panel 111, or a string of such PV panelsconnected in series, and contains a DC-DC power converter circuit 112and a control circuit 113. The control circuit has means 114 formeasuring the voltage of rails 101 and 102, and a coupling circuit 115capable of coupling an AC signal between the control circuit and the DCrails, in order to transmit and receive data by powerline carriercommunication (PLC). PLC communication allows devices connected to thebus (e.g. 110, 120) to communicate setpoints and operational parameters.Additionally it allows users to gather information and make changes tocomponent setpoints from any device on the bus.

Other energy units are optionally connected to the rails 101 and 102 inparallel with converter 110, for instance energy unit 120, which acceptspower from a wind turbine 121, typically as three-phase ‘wild’ AC,(variable voltage, variable frequency), and contains an AC-DC converter122 capable of converting the wind turbine output to regulated DCvoltage for connection to rails 101, 102. Converter 120 contains acontrol circuit 123, voltage sensing means 124, and a coupling circuit125 for communication, as described above.

In some embodiments, power line carrier communication is implemented bysuperimposing an AC signal on the DC bus, for instance by means of 144kHz frequency shift keying (FSK). With reference to the lower right ofFIG. 1B, a coupling circuit comprises a coupling capacitor 116,transformer 117, and communication controller chip 118. A high frequencysignal is generated by controller chip 118, and is modulated by alow-baud rate data signal and the result is amplified. This modulatedhigh-frequency current may be injected onto the bus through isolatingtransformer 117 and coupling capacitor 116. For reception, thetransmitted signal passes through a receiving-end isolating capacitore.g. 116 and is filtered appropriately to retrieve the data stream.Integrated circuits for performing FSK for PLC communication arecommercially available, for instance the PLM-1 IC from Ariane Controls.The AC communications signal may be injected onto the DC rails indifferential mode between the rails or in common mode with respect toground.

Because each rail of the bus carries only about half of the total systemvoltage relative to earth ground (e.g. +/−190V in a 380V system), thepotential for electric shock is much reduced, while still achieving thehigh transmission voltage that enables efficient power transmission.Additionally, the split-rail approach enables a single-stage design forinverter 140, by eliminating the need for an input conversion stage.This significantly reduces the cost and power loss of the inverter.Finally, the split-rail architecture makes it possible to detect groundfault conditions.

One unique attribute of the energy distribution system of FIG. 1A is theabsence of a central controller or a master bus regulator. Stability ofthe bus is instead achieved by the collective action of the convertermodules connected to it. Each energy unit connected to the bus regulatesits current onto the bus according to the bus voltage as measured (e.g.by voltage sensing network 114), with a strategy that collectivelyeffects a voltage-regulated system. This voltage-controlled powerregulation enables plug-and-play operation with arbitrary combinationsof sources and loads.

Three general types of energy units may be defined for use with theenergy distribution system proposed here. Primary Source Units (PSU)supply electric power to the bus, for instance from wind turbines orsolar panels. Load Units (LU) draw power from the bus, and EnergyStorage Units (ESU) are bi-directional, and may draw power from the bus,or supply power to it.

With reference to FIG. 2A, a Primary Source Unit is permitted to supplyas much current as possible (up to its rated limit) when the bus voltageis below a source threshold 201, and then smoothly curtails its outputas bus voltage increases beyond that threshold, such that no current isdelivered to the bus when the bus voltage reaches a higher source cutoffvoltage 202. In other embodiments, the PSUs regulate to a maximumvoltage without the linear ramp to zero power. Exemplary types of PSUinclude without limitation converters for solar photovoltaic modules,wind turbines, fuel cells, hydroelectric installations, fossil-poweredheat engines, or rectifiers (where the grid acts as a virtual energysource).

With reference to FIG. 2B, a Load Unit is permitted to draw current fromthe bus up to its rated limit when the bus voltage is above a loadthreshold voltage 210, but must curtail its current consumption belowthe hysteretic cutoff voltage 211. The load current consumption may beincreased gradually above the threshold voltage, or it may be on/off.Load units may be programmed for relative priority, as may be beneficialwhere more than one Load Unit is connected to a single network. A lowerpriority LU is programmed with a high load threshold voltage, such thatit curtails its load before a LU with a lower load threshold voltage.Exemplary types of energy devices that a LU might connect includewithout limitation lighting, HVAC loads, appliances, servers and otherdata processing equipment, and grid-tied inverters (where the grid actsas a virtual load).

With reference to FIG. 2C, an Energy Storage Unit sinks current from thebus at high bus voltage, but (optionally) sources current to the bus atlow bus voltage. When acting like a load, the ESU transfers energy toits storage medium, and when acting like a source it supports the loadson the network. In this way, excess energy may be stored for later use.Exemplary types of energy device that an ESU might connect includewithout limitation battery banks, electric vehicles, flow batteries, andelectrolyzer/fuel cell systems, as well as bi-directional grid-tiedinverters (where the grid acts as virtual storage—i.e. as both a sourceand a load). Importantly ESUs must follow the prescribed load curve andshould not exhibit the on/off behavior of some forms of load unit.

In some embodiments, the voltage-controlled power regulation enablesmultiple identical energy devices to be connected to the bus inparallel. For example, if two Energy Storage Units are attached to thebus in parallel with the same load threshold settings, they will sharethe available power equally. This increases scalability of renewableenergy systems, reduces the economic barrier to initial systeminstallation and avoids obsolescence as the system is scaled up.Parallelability also reduces the number of models that a manufacturermust produce. For example, instead of offering 1 kW, 2 kW and 4 kWconverter models, the converter manufacturer may elect to produce only a1 kW model. Consumers can purchase 1, 2 or 4 of them. The 1 kW modelscould possibly be produced at significantly lower cost because of thehigher volume.

One of the major drawbacks to previous DC distribution systems has beenthe requirement for expensive disconnect switches capable of breaking asustained electrical arc. In some embodiments of the present invention,means are provided to enable converters connected to the bus to survivea sudden drop in impedance, for example an accidental or intentionalshort circuit. With reference to FIG. 3, a source converter 310 isequipped with a series switch 320, for instance a solid state switchsuch as a Field Effect Transistor (FET), and a differential sensingamplifier 321 coupled to a circuit that controls the gate of the FET320. In normal operation a blocking diode 326 is forward biased andkeeps the amplifier input voltage sufficiently low such that the switchremains on to connect the device to the bus. However, in the event of asudden drop in bus impedance, the rising current across the onresistance of FET 320 creates a voltage across the switch which isblocked by blocking diode 326, preventing overvoltage to the amplifiercircuit. The increased voltage across the switch 320 reverse biases theblocking diode 326 which was previously loading the balance circuit 324which in general is pulling the negative terminal of the amplifier 321high enough to cause a disconnect. With the blocking diode no longerpulling down the balance circuit, the amplifier switches state and turnsoff the switch 320. The filter circuit 323 prevents spurious activationof the disconnect circuit. The compensation circuit 325 ensures that theswitch does not glitch and retains it's off state indefinitely. In thisembodiment the entire circuit is referenced to the negative bus rail tofacilitate driving the switch. In order to operate the disconnectcircuit from the logic circuit 113 an optoisolator switch 327 isemployed which connects a power supply 328 that is referenced to thelower side of the switch 320. Reset circuit 322 causes the amplifier tolatch on when initially powered up, but critically does not prevent thedisconnect circuit from turning the switch off. For the logic circuit113 to turn the switch back on after it has been activated(disconnected), it must first turn the power to the circuit off bydeactivating the optoisolator 327 and then repowering it.

This system of disconnect operation is advantageous for two reasons.First, the microprocessor never directly turns the switch on which ifcommanded directly for even a short time (for example, the time betweentwo instruction cycles on the microprocessor) could damage the powerconverter. In this circuit protection of the power converter is neverdisabled. Second, the expensive optoisolated gate driver typicallyemployed is replaced with one inexpensive optoisolator and only oneoutput pin on the microprocessor is required.

Converter 310 may be designed to limit its current output in the eventof a short circuit on the bus, disconnect from the bus, and shut downcompletely within a short time after a short has occurred. This protectsthe converter 310 from damage from excessive current, without need for aphysical disconnect capable of breaking a DC arc, and enables the bus tobe shut down intentionally with a short circuit if desired. Previously,shutdown of a HVDC bus required that every attached device beopen-circuited from the bus with switchgear capable of breaking a DCarc. This topology is significantly less expensive, and it provides amethod for any single device to effect a bus shutdown by shorting thebus conductors—a novel and beneficial ground fault protection system.This technique means that any ground fault or other potentiallyhazardous situation can be handled by any device attached to the bus.

Several converter designs may be suitable for use with the busarchitecture of the present invention. With reference to FIG. 4 a sourceconverter 451 for an array of one or more solar PV modules comprises asingle-phase boost converter, comprising a source capacitor 400, a boostinductor 401, a boost switch 402, a boost diode 403, a bus sidecapacitor 404, a disconnect switch 405, a sensor network 406, a DigitalSignal Processor (DSP) 407, and a gate drive circuit 408. In operation,the DSP 407 drives the boost switch 402 with a pulse-width modulation(PWM) signal via the gate drive circuit 408, storing energy in inductor401 and driving current through boost diode 403 and onto the bus. TheDSP 407 monitors bus voltage and current, and performs Maximum PowerPoint Tracking to ensure the greatest possible output from the PV array.

FIG. 4 illustrates an example design of an Energy Storage Unit 452 forinterfacing with a battery bank, for instance at a lower operatingvoltage than the bus. The circuit comprises a storage side capacitor420, an inductor 421, a boost switch 422, a buck switch 423, a bus sidecapacitor 424, a disconnect switch 425, a sensor network 426, a digitalsignal processor 427, and a gate drive circuit 428. When storing energyin the battery, the DSP 427 provides a pulse-width-modulated signal tothe buck switch 423, storing energy in inductor 421 and driving currentinto the battery bank When delivering stored energy to the bus, the DSP427 provides a PWM signal to the boost switch 422, storing energy ininductor 421 and driving current through the buck switch 423 and ontothe bus. Disconnect switch 425 may be used to isolate the ESU converterfrom the bus in case of a short circuit.

FIG. 4 illustrates an exemplary design of bidirectional grid-tieinverter ESU 454, comprising split input capacitance 440, half bridges441 and 442, output filter networks 443 and 444, sensor network, DSP,and gate drive circuit. In operation, the DSP drives PWM signals to halfbridges 441 and 442 through gate drive circuit 447, in order to producea sinusoidal output current with the desired energy output correspondingto the voltage of the bus.

In order to perform as intended, stability of the bus voltage must bemaintained both at steady state, and also dynamically for small-signalperturbations. Steady state system stability requires that the DC busvoltage settle to a value that is both appropriate as a distributionvoltage, and also indicative of the system's energy availability (i.e.monotonic and non-hysteretic) to enable smart load shedding and otheradvanced functionality. This aspect of the disclosed bus is achievedwith the Voltage Controlled Power Regulation (VCPR) curves shown in FIG.2A-C. The linear ramp portions of these curves dictate the location ofthe steady state transmission voltage for a given energy availabilitylevel. With reference to FIG. 5A, a 3000 W Energy Storage Unit (e.g.inverter) has the VCPR profile shown as a dashed line. A Primary SourceUnit (e.g. a PV source) has a 2200 W output, shown as the solid line.The steady state operating voltage of the DC bus is determined by thepoint where the two lines cross (near 398V). In FIG. 5B the PSU is stilloperating at 2200 W, but the ESU is smaller, with only 1500 W ofcapacity. The operating voltage is higher, near 406V, and the highervoltage signals the PSU to curtail its output, while also indicating toother components attached to the bus that more energy is available thanis being consumed. This could result in an additional load activatingthat was not previously on. In FIG. 5C a 1500 W constant load has beenturned on. In this scenario the bidirectional 3000 W inverter suppliesthe necessary power to maintain the load at approximately 384V.

The bus must also maintain small-signal or dynamic stability. This isaccomplished by segregation of impedances in the frequency domain. Withreference to FIG. 6A, the microgrid can be modeled as one or moresources connected in parallel with one or more sinks. In this analysisthe power converters are modeled as current sources with Nortonimpedances, but voltage sources with Thévenin impedances would beequivalent. For current source Ia1 the Norton impedance Za1 includes allof the system dynamics including the power converter plant, passivefilter components, and the microprocessor control loop. Similarly theimpedances Zx1 sum the effects of all of the transmission lineparasitics (resistance, inductance, capacitance to ground, andcapacitance to each other).

This microgrid architecture is designed to accommodate unspecifiednumbers of sources and sinks, but each additional power converterchanges the line impedance that every other power converter observes. Sothe disclosed approach is to make a specification per unit power andthen stipulate that sources cannot overpower sinks by more than a givenoverload factor (for example 2). This overload factor is then built intothe microgrid specification such that reduced impedance due toadditional power converters does not impact system stability.

Since the specification is per unit power, analysis can be performed onisolated sections (for example one source and one sink) and thensuperposition used to analyze the complete system. With reference toFIG. 6B, the sink in the isolated section has been replaced with itsequivalent small-signal impedance Zb′. This impedance includes theeffects of the sink's control algorithm as well as the power converterplant and passive filter components (Zb).

From this simplified section, a block diagram (shown in FIG. 6C) is usedto calculate the system dynamics. Nichols theorem says that if the loopgain plotted on a polar plot does not encircle the point (−1, 0) thesystem will be stable. With regard to a standard Nichols plot, the pointwhere the loop gain crosses the x-axis informs about the gain marginwhile the phase angle where the loop gain crosses the unit circleinforms about the phase margin.

The loop gain of the block diagram in FIG. 6C is Za divided by Zb′+2*Zxor equivalently, the output impedance of the source divided by the inputimpedance (plus transmission impedance) of the sink As long as theoutput impedance is less than the input impedance the system will bestable since the unit gain circle on the Nichols plot has not beenreached. At the point when the output impedance of the source is equalto the input impedance of the sink (plus the transmission impedance),the difference in phase (source output impedance phase minus sink inputimpedance phase) should be less than 180 degrees.

To ensure that these conditions are met, keepout areas are defined forthe power converter's impedances. FIG. 7 shows gain and phase plots(Bode plots) for an example source output impedance magnitude 701 andphase 711 and sink input impedance magnitude 702 and phase 712 versusfrequency. The magnitude keepout area 705 sets a lower limit for theinput impedance magnitude 702 and an upper limit to the output impedancemagnitude 701. The magnitude keepout area 705 extends to the magnitudekeepout frequency 704 guaranteeing that the output source impedance isnever more than the input source impedance and hence the system isstable in this frequency range.

Beyond the magnitude keepout frequency 704, the phase of the sourceimpedance 711 must be less than 180 degrees greater than the phase ofthe sink impedance 712. This is assured by the phase keepout areas 715and 716. The upper phase keepout area 715 defines the maximum allowablephase of the source impedance 711. The lower phase keepout area 716defines the minimum allowable phase of the sink impedance 712. The phasekeepout areas begin at frequency 703 which is necessarily less than themagnitude keepout area frequency 704 to ensure that either the ratio ofthe magnitudes is less than 1 or the difference in phase is less than180 degrees and the Nichols criterion is met for all frequencies. Again,these impedances are compared against the specification as per unitpower (typically per kilowatt) and consequently the system scales toincorporate more power converters. The width of the magnitude keepoutareas accounts for mismatch of sinks and sources as well as unforeseenimpedances found in any real-world application. The phase keepout areasare less than 180 degrees apart for a similar margin of safety.

In some embodiments, robust components may be used for primary sourcegeneration equipment situated in challenging environments. This benefitcomes from the use of DC/DC equipment at the point of generation ratherthan DC/AC conversion equipment. Specifically, DC/AC converterstypically require the use of heat-sensitive components with limitedlifetimes—especially electrolytic capacitors—in order to store thesteady-state flow of input energy from the source, while deliveringpulses of energy at 120 Hz as required to produce 60 Hz AC power. Thesecomponents are not necessary in DC/DC applications, greatly reducing thethermal management challenge of electronics mounted directly to PVmodules. The small amount of capacitance needed can be supplied withrobust ceramic or film-based capacitors.

1. A system comprising: a first energy unit having a voltage convertercoupled to an energy device and adapted to couple to a pair of split DCrails, a second energy unit having a voltage converter coupled to anenergy device and adapted to couple to the rails, wherein the energyunits sense the voltage on the DC rails and regulate the rail voltage bymeans of autonomous current responses that create the aggregate effectof controlling the rail voltage in cooperation with other units coupledto the DC rails.
 2. The system of claim 1 wherein at least one of theenergy units has communications circuitry that transfers data to otherenergy units coupled to the DC rails by coupling an AC signal to the DCrails in either differential mode with respect to each other or commonmode with respect to ground.
 3. The system of claim 2 where thecommunications data is encoded by Frequency Shift Keying.
 4. The systemof claim 1 and further comprising a storage unit having a voltageconverter coupled to a storage device and sourcing or sinking power tothe DC rails as a function of DC rail voltage.
 5. The system of claim 4wherein the storage device is the utility grid.
 6. The system of claim 1wherein the split DC rails have a substantially equal but oppositepolarity voltage.
 7. The system of claim 7 where the potential of eachconductor relative to ground potential is in the range of +/−180-210volts.
 8. The system of claim 1 energy units source or sink power fromor to the DC rails in accordance with a priority algorithm.
 9. Thesystem of claim 1 wherein the voltage converters are adapted toautomatically disconnect from the DC rails without damage when the DCrails are shorted together.
 10. The system of claim 9 wherein the energyunits short the DC rails in response to a fault or to stop or prohibitfunction of the system.
 11. The system of claim 1 and further comprisinga ground fault detector to couple to the split rails of the DC bus. 12.The system of claim 1 wherein one or more of the energy units is coupledto a source of solar electricity.
 13. The system of claim 1 wherein oneor more of the energy units is coupled to a wind-powered source ofelectricity.
 14. The system of claim 1 wherein one or more of the energyunits is coupled to at least two different sources of electricity.
 15. Adevice comprising: an energy unit coupled to an energy device andadapted to couple a pair of split DC rails, and a controller to sensethe voltage on the DC rails and regulate its output current response bymeans of an autonomous current response that creates the aggregateeffect of controlling the rail voltage in cooperation with other unitscoupled to the DC rails.
 16. The device of claim 15 wherein the energyunit has a communications module to communicate with other energy unitscoupled to the DC rails.
 17. A method comprising: sensing a voltage viaan energy unit coupled to a pair of split DC rails; determining fromthat voltage a current response; implementing the current response andrepeating the above steps to create the aggregate effect of controllingthe rail voltage in cooperation with other units coupled to the DCrails.
 18. The method of claim 17 wherein magnitude areas are used tospecify the current response.
 19. The method of claim 17 wherein phasekeepout areas are used to specify the current response.
 20. The methodof claim 17 wherein magnitude and phase keepout areas are used tospecify the current response as a function of per unit power.